(1) Field of the Invention
The present invention relates to a ceramic honeycomb structure used as a heat accumulator, etc.; a regenerative thermal oxidizer using the ceramic honeycomb structure; and a process for producing the ceramic honeycomb structure.
(2) Description of Related Art
In recent years, there has been a problem that the volatile organic compounds (hereinafter referred to as VOC) and unburnt gas contained in the exhaust gases from plants, etc. incur forest exhaustion by acid rain or injure the health of people living in the neighborhood of the plants, etc. In order to remove such VOC and unburnt gas by combustion, regenerative thermal oxidizer (hereinafter referred to as RTO), regenerative catalytic oxidizer (hereinafter referred to as RCO), etc. are in use. Incidentally, removal of the VOC and unburnt gas in exhaust gas from plant is generally called xe2x80x9cdeodorizationxe2x80x9d, and a gas containing VOC and unburnt gas, before deodorization is herein referred to as xe2x80x9cpre-deodorization gasxe2x80x9d.
RTO or RCO has therein a heat-accumulating unit which can conduct heat exchange with a gas. In RTO or RCO, therefore, the heat generated by combustion of VOC and unburnt gas is utilized for heating of the next (fresh) pre-deodorization gas, which enables (1) reduction in deodorization cost, (2) energy saving and (3) reduction in global warming speed, resulting from release of smaller amount of combustion heat.
In FIG. 1 is shown a three-chamber type RTO which is the most ordinarily used RTO. In FIG. 1, a three-chamber type RTO 1 has three heat exchange chambers 2a, 2b and 2c; and each of the heat exchange chambers 2a, 2b and 2c communicate with a single combustion chamber at the top. Each heat exchange chamber 2 has, at the bottom, a gas-feeding pipe 4 and a gas-discharging pipe 5. In each heat exchange chamber 2 is provided a heat-accumulating unit 7 constituted by a honeycomb structure. At the ceiling of the combustion chamber 3 is provided a burner 6.
In the three-chamber type RTO 1, having the above constitution, a pre-deodorization gas is fed into either one of the three heat exchange chambers 2a, 2b and 2c via the gas-feeding pipe 4, and passes through the heat-accumulating unit 7 of the heat exchange chamber, and enters the combustion chamber 3. At this time, heat exchange occurs between the pre-deodorization gas and the heat-accumulating unit 7, whereby the pre-deodorization gas is heated to near the combustion temperature. In the combustion chamber 3, the VOC and unburnt gas in pre-deodorization gas are burnt by the burner 6. The gas after deodorization (hereafter referred to as xe2x80x9cpost-deodorization gasxe2x80x9d) passes through the other two heat exchange chambers 2 and are released into air via the gas-discharging pipe 5. At this time, the heat generated by combustion of VOC and unburnt gas is partly adsorbed by the heat-accumulating units 7 of the other two heat exchange chambers 2. The heat exchange chamber 2 into which the pre-deodorization gas is fed and the other two heat exchange chambers 2 which discharge the post-deodorization gas, are switched at given intervals, and the above operation is conducted continuously, whereby the heat generated by combustion of pre-deodorization gas is used to heat the next (fresh) pre-deodorization gas. In the case of RCO, a catalyst is supported on the top of each heat-accumulating unit 7, whereby the activation energy necessary for the combustion of VOC and unburnt gas is lowered and the deodorization cost is reduced further.
As the above-mentioned heat-accumulating unit of RTO and RCO, a honeycomb structure is used. It is because the honeycomb structure has a large surface area per unit volume and accordingly is superior in heat exchange property, can give a small RTO and can give a small pressure loss. Incidentally, xe2x80x9choneycomb structurexe2x80x9d refers to, for example, a structure 13 shown in FIG. 2, wherein a large number of cells 11 are surrounded by partition walls 12. Since the structure 13 has a large contact area with a gas fed from a plurality of cell openings at the one end, the heat exchange between the honeycomb structure 13 and the gas is conducted efficiently. As the material of the honeycomb structure 13 constituting the heat-accumulating unit, there is generally used a ceramic superior in heat exchange property. Ordinarily, a honeycomb structure similar to that used as a carrier for catalyst for automobile exhaust gas purification is used as the heat-accumulating unit of RTO.
The honeycomb structure used as a carrier for catalyst for automobile exhaust gas purification is highly porous in many cases in order to maximize the amount of catalyst loaded thereon; therefore, when it is used as the heat-accumulating unit of RTO, there have been problems such as (1) low heat exchangeability due to insufficient heat accumulation capacity and (2) adsorption of VOC and unburnt gas inside pores and resultant release of pre-deodorization gas into air. There has also been a problem that the small bulk specific gravity of honeycomb structure gives rise to floating of honeycomb structure in heat exchange chamber by pressure of pre-deodorization gas, inviting reduction in heat exchange efficiency.
To alleviate such problems, it is considered to use, as the heat-accumulating unit of RTO, a porcelain of low porosity. In this technique, however, there has generally been a problem that use of a porcelain of small cell pitch to increase its geometrical surface area (hereinafter referred to as GSA) and accordingly increase its heat exchange area, makes large the contraction during firing and causes cracking easily.
In view of the above situation, the present invention aims at providing a ceramic honeycomb structure which has a sufficient heat accumulation capacity, which hardly causes adsorption of VOC, etc. or floating due to the pressure of gas, which is resistant to rupture, and which has a large GSA; a RTO using the honeycomb structure; and a process for producing the honeycomb structure.
According to the present invention, there is provided a ceramic honeycomb structure having an open frontal area of 50% to 85%, a porosity of 0.1% to 10%, and a proportion of the volume of pores of 1 xcexcm or larger in diameter, in total pore volume, of 20% or more.
The ceramic honeycomb structure of the present invention preferably has a thermal expansion coefficient of 3.0xc3x9710xe2x88x928/xc2x0 C. or less in the flow direction of cell, an average pore diameter of 0.01 xcexcm or more, a partition wall thickness of 100 xcexcmm to 800 xcexcm, and a geometrical surface area of 0.8 mm2/mm3 or more. In the ceramic honeycomb structure of the present invention, the porosity is preferably 7.5% or less and the main crystalline phase is preferably made of cordierite. The ceramic honeycomb structure of the present invention may be a heat accumulator.
According to the present invention, there is further provided a regenerative thermal oxidizer (RTO) or regenerative catalytic oxidizer (RCO) using a plurality of the above ceramic honeycomb structures.
According to the present invention, there is furthermore provided a process for producing the above ceramic honeycomb structure, which comprises adding at least a molding aid and a solvent to a raw material mixture comprising a ceramic material and a ceramic precursor material, kneading the resulting mixture to obtain a molding material, extruding the molding material into a honeycomb-shaped material, and drying and firing the honeycomb-shaped material, wherein
the proportion of the ceramic material in the raw material mixture is 50 to 95% by weight,
the average particle diameters of the ceramic material and the ceramic precursor material are each in a range of 3 to 25 xcexcm,
the average particle diameter of the raw material mixture is in a range of 5 to 20 xcexcm, and
when the average particle diameter of the ceramic material is Da and the average particle diameter of the ceramic precursor material is Db,
Da is in a range of 3 xcexcm to less than 5 xcexcm, and Da and Db satisfy Daxe2x89xa60.6xc3x97Db, or
Db is in a range of 3 xcexcm to less than 5 xcexcm, and Da and Db satisfy Dbxe2x89xa60.6xc3x97Da, or
Da and Db are each in a range of 5 xcexcm to 15 xcexcm, or
Da is in a range of 15 xcexcm to less than 25 xcexcm, and Da and Db satisfy 0.6xc3x97Daxe2x89xa7Db, or
Db is in a range of 15 xcexcm to less than 25 xcexcm, and Da and Db satisfy 0.6xc3x97Dbxe2x89xa7Da.
In the production process of the present invention, the proportion of the ceramic material in the raw material mixture is preferably 75 to 95% by weight. In the production process of the present invention, each of the ceramic material and the ceramic precursor material may consist of first particles having an average particle diameter of 15 xcexcm to less than 25 xcexcm and second particles having an average particle diameter of 60% or less of that of the first particles, and the proportion of the second particles in each material may be 5% by weight to less than 50% by weight.
Also in the production process of the present invention, at least part of the surfaces of the particles constituting the ceramic precursor material may be coated with a molding aid or a solvent, and the ceramic material may be a material obtained by firing the ceramic precursor material. In this case, the main crystalline phase of the ceramic material is preferably made of cordierite.